A reduction-oxidation (redox) flow battery is an electrochemical device which converts chemical energy stored in a battery to electrical energy via reverse redox reactions. When depleted, the chemical energy in the battery is restored by applying an electrical current to induce reverse redox reactions.
In general, the redox flow battery includes negative and positive electrodes contained within a battery cell stack. These electrodes take part in electrochemical reactions responsible for storing and releasing chemical energy in the battery, and thus affect battery performance and overall costs. In practice, a plurality of battery cells are stacked together in electrical series to produce a desired voltage or power level. Perimeter spacers may be placed in between stacked cells to provide a cushion between battery cells while permitting electrical connectivity of the cell stacks. Each stack of cells is compressed between two rigid endplates using a compression system that aims to provide an adequate force to seal the cell stack and compress an active area of the battery cell stack, without overly stressing components. The compression system also tries to accommodate changes in stack height caused by thermal expansion and contraction of the cell stack during operation, although these objectives may compromise the balance of sufficient but not excessive compressive force.
One example compression system is presented by Blanchet in U.S. Pat. No. 6,413,665. The system comprises spring and linkage mechanical assemblies used in conjunction with tie rods and bars to compress a fuel cell stack. The linkage mechanism contains a lever and pins to transfer a compression load imposed by the spring assembly through the tie rods and bars (attached to an end plate strapped at the bottom of the cell stack) to the fuel cell stack. Further, the spring assembly contains a plurality of springs configured to provide a decreasing load profile as the fuel cell stack is compressed by cell consolidation. Other cell stack compression systems may include large coil springs attached to an end plate strapped around a battery cell stack of a flow battery. The coil springs are designed to transfer a compression load from a link mechanism to the battery cell stack.
However, the inventors have recognized potential issues with such compression system for cell stacks. For example, compression loads imposed only at one end of the cell stack may generate asymmetric loading of the cell stack and may generate structural degradation prematurely. Further, issues related to the compressive loads and expansion, as well as the interactions therebetween, can be particular to flow battery systems.
As another example, coil springs designed to provide loading at the base of the springs and around the periphery of the cell stack may generate non-uniform loading of the cell stack, inducing deflections larger than permitted. Overly large deflections may cause unstable conditions within the battery cell stack affecting performance of the flow battery.
The inventors herein have recognized the above issues and developed various battery cell stack compression systems. In one example, a compression system comprising a tie rod assembly may be used in conjunction with reinforcement bars, a plurality of springs, such as leaf springs, and fulcrums to apply a compression load on pressure plates attached to a battery cell stack of a flow battery. The tie rod assembly may be adjustable to produce a desirable compression load which may be transferred through the leaf springs to the battery cell stack. A pair of fulcrums positioned behind each leaf spring may be configured to redirect the compression load exerted by the tie rod assembly to an active area of the cell stack to maintain uniform loading on the cell stack. By redirecting the compressive load imposed on the cell stack, the compression system may reduce non-uniform loading of battery cell stack while keeping deflections in the cell stack system to threshold levels.
The approach described here may confer several advantages. For example, the compression system may be designed to provide uniform loading on the battery cell stack under a wide range of operating conditions. Further, the compression system can be adjusted to allow for expansion and contraction of the cell stack during operation while keeping deflection of the cell stack system within allowable levels and minimizing overall costs.
The above discussion includes recognitions made by the inventors and not admitted to be generally known. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.